EXISTING HOMES

Spray Foam Insulation  requires the removal of existing insulation for the following reasons:

-The section in the building code that addresses this is R806.4 where it states that no vapor retarders are to be installed on the attic floor.  This most typically refers to the kraft facing of a fiberglass batt insulation.  So, according to the Florida Building Code, the very minimum that needs to be removed would be the kraft facing.  The reasoning behind this is that we want the moisture levels of the attic and living area to equalize.  Since we are not directly conditioning the attic space with ductwork, we need to ensure that there are not high moisture conditions in the attic. 

-From a building science standpoint, when open cell foam is installed in an attic, the chemical reaction introduces moisture from the water in the chemical breaking down.  This can result in moisture being absorbed by the existing fiberglass insulation, which then has to be removed from the HVAC unit.  This can cause a slight rise in energy bills for the first month or two because of the added latent load to the HVAC unit. 

-From a health standpoint, existing insulation, especially in older homes, can be quite dirty, dusty, wet, and possibly filled with caustic animal excrements.  With a closed attic assembly, these items are then closed into the thermal envelope.

-Fiberglass insulation is not an air barrier.  Therefore, if the insulation is not completely pulled back or removed around the perimeter of the house, an installation crew runs the risk of not properly sealing the attic (if the foam is simply sprayed down to the fiberglass.)

-One final concern is that the catalysts in the foam (amines), have a certain smell that needs to be exhausted either through ventilation during spraying or through the HVAC system. 
If the existing insulation is not removed, some of these smells can be trapped in the existing insulation, taking much longer to be removed.

According to the Florida Building Code (see point 2 in R806.4):

R806.4 Unvented attic assemblies. Unvented attic assemblies shall be permitted if all the following conditions are met:

1. The unvented attic space is completely contained within the building thermal envelope.
2. No interior vapor retarder is installed on the ceiling side (attic floor) of the unvented attic assembly.
3. Where wood shingles or shakes are used, a minimum continuous 1/4 inch (6 mm) vented air space separates the shingles or shakes from the roofing underlayment.
4. One of the following shall be met, depending on the air permeability of the insulation under the structural roof sheathing:

a. Air-impermeable insulation only. Insulation shall be applied in direct contact to the underside of the structural roof sheathing.
b. Air-permeable insulation only. In addition to air-permeable insulation installed directly below the structural sheathing, at least R-5 rigid board or sheet insulation shall be installed directly above the structural roof sheathing for condensation control.
c. Air-impermeable and air-permeable insulation. At least R-5 air-impermeable insulation shall be applied in direct contact to the underside of the structural roof sheathing for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation.

In regards to performance of foam vs. cellulose and fiberglass:

The main benefit from spray foam insulation comes from the air barrier qualities.  The r-value test is based solely on the conduction method.  This means that a laboratory tests fiberglass insulation that has not been compressed (note that fiberglass is compressed in typical packaging), a lab sees how much heat transfers from one side of the insulation to another.  This is done in a vacuum with no air currents and no outside heat sources.  In Florida, we have high wind conditions as well as the sun that beats down on our roofs with extreme radiant heat.  When we take into effect that the foam is blocking the radiant heat from the sun and the hot humid air from coming into our homes, we can see that foam in conjunction with a nonvented attic assembly performs much better than typical fiberglass insulation because these performance qualities are not measured by the r-value test.  Attached is a test performed by Accu Test Labs which shows the differences between different insulation products.  Also note in section R806.4 that fiberglass insulation cannot be used alone in a nonvented attic assembly because it is not an air barrier (R806.4.4.a).

Another benefit of spray foam is that it is a perfect application every time.  There is no cutting or stuffing of our products like you typically see with fiberglass.  Fiberglass batts only come in certain sizes.  If the project to be insulated has studs or trusses that are not perfectly spaced (which is usually the case), then you will see air gaps on either side of the fiberglass, which will severely reduce the effective r-value.  We also see that when trying to insulate around pipes or wiring, fiberglass insulation is typically cut or compressed to fit, reducing its performance r-value.  For blown insulation, we typically see that the high wind conditions in Florida will blow that insulation away from the perimeter of the house when the wind enters the soffits.  This effectively leaves an uninsulated band around the perimeter of the home, drastically reducing the effectiveness of the insulation.  Attached is a picture and additional documents showing the thermal deficiencies of a typically installed fiberglass home.